1. Field of the Invention
Embodiments disclosed herein relate generally to control systems for subsea blow out preventers (BOPs). In particular, embodiments disclosed herein relate to systems for providing backup or augmented functions in addition to a BOP control system.
2. Brief Description of Related Art
Subsea drilling for oil and gas typically involves the use of a vessel, which can be, for example, a drill ship or a platform, on the surface of the sea, with a riser extending to near the sea floor. The bottom end of the riser is attached to a lower marine riser package, which contains, among other things, control pods intended to control components of the drilling system near the sea floor. Below the riser is typically positioned a lower stack, which includes a blowout preventer (BOP) mounted to a wellhead. The drilling pipe extends from the vessel at the sea surface, through the riser to the bottom of the sea, through the BOP, and through the wellhead into a wellbore to the oil producing formation.
One purpose of the BOP is to act as a failsafe mechanism to prevent oil and gas from escaping from the wellbore into the environment. To accomplish this task, the BOP typically includes a plurality of rams. Some rams have elastomeric seals and are designed to close around the drill pipe if needed to seal the annulus around the pipe. That way, if an unexpected pressure surge tries to force oil and gas from the wellbore through the annulus, the BOP can close to prevent a spill. Other rams are known as shearing rams, including blind shear rams, and are designed to cut through drill pipe and other items extending into the wellbore to completely seal the wellbore from the surrounding environment.
Because of the safety functions played by the BOP in a subsea drilling operation, it is necessary to ensure that all BOP control systems and components are functioning properly, and to provide redundant backup systems in case of a failure. Accordingly, added redundancy and monitoring capability, such as in the form of a safety instrumented system, is beneficial.
In addition, as subsea drilling extends into deeper waters, pressures at the sea floor, where the BOP is located, increase. With higher pressures, there are greater consequences if a well leaks, and the BOPs themselves require design modifications to ensure safety. Accordingly, new safety instrumented systems for backing up the BOP control system, as well as monitoring its function, are needed.